The goal of the proposed research is to understand how the nervous system orchestrates complex behavior. Complex behaviors require the temporal coordination of independent neural circuits. Despite widespread recognition that the action of neurotransmitters and ion channels fine tune the output of neural circuits, there is a surprisingly limited understanding of how the nervous system directs sequential activation and inhibition of assemblies of neurons to orchestrate behavior. Alterations in the neurotransmitter systems and ion channels have long been implicated in the etiology of a variety of neurological disorders, underlining the need to develop effective approaches that can directly relate the coordinated activity of specific neuronal circuits to complex behaviors. To elucidate how the nervous system orchestrates complex behaviors at the molecular and neural level we are studying the C. elegans escape response, which is a highly orchestrated motor sequence that requires sensory processing, decision-making and the temporal coordination of independent motor programs. Our analysis has unraveled how presynaptic voltage-gated Ca2+ channels (CaV2) and monoamines temporally coordinate different phases of the response through synaptic activation of fast-acting ionotropic receptors, and extra-synaptic activation of slow-acting metabotropic receptors. We will use Ca2+ imaging and optogenetics to define temporal and causal relationship between neuronal activity and the sub-motor programs of the escape response. To determine how independent motor programs are linked in the execution of a compound motor sequence we will study how reversals are coupled to turning behavior during the escape response. We will use mutant analysis to test if coincidence detection or post-inhibitory rebound mechanisms account for sequential activation of these sub-motor programs. Since neuromodulators precisely regulate synaptic activity through the inhibition of presynaptic voltage-gated Ca2+ channels (CaV2), we will define novel CaV2 signaling components that regulate CaV2 channels in circuit function. Our C. elegans CaV2 gain-of-function mutant provides the first invertebrate model for familial hemiplegic migraine and provides novel tool to modify circuit performance in the escape response. The mechanisms that organize activity in the escape circuit of C. elegans will illuminate similar mechanisms that orchestrate complex behaviors in more complex animals including humans. We expect that our studies will have a major impact on our understanding of how neuromodulators and voltage-gated-calcium channels affect circuit function in behavior and neurological disorders, and will provide new molecular targets and strategies for the treatment of these diseases.